There have been proposed a number of amorphous silicon (hereinafter referred to as "A-Si") films for use as an element member in semiconductor devices, image input line sensors, image pickup devices or the like. Some such films have been put to practical use.
Along with those amorphous silicon films, there have been proposed various methods for their preparation using vacuum evaporation technique, heat chemical vapor deposition technique, plasma chemical vapor deposition technique, reactive sputtering technique, ion plating technique and light chemical vapor deposition technique.
Among those methods, the method using heat chemical vapor deposition technique (hereinafter referred to as "CVD method") had been tried once in various sectors, but nowadays it is not used because elevated temperatures are required and a practical deposited film can not be obtained as desired.
On the other hand, the method using plasma chemical vapor deposition technique (hereinafter referred to as "plasma CVD method") has been generally recognized as being the most preferred and is currently used to manufacture amorphous silicon films on a commercial basis.
However, for any of the known A-Si films, even if it is such that it is obtained by plasma CVD method, there still remain problems unsolved relating to the film's characteristics, particularly electric and optical characteristics, deterioration resistance upon repeated use and use-environmental characteristics. The solutions to these problems must correlate with its use as an element member for the foregoing devices and also for other points such as its homogeneity, reproducibility and mass-productivity.
Now, although the plasma CVD method is widely used nowadays as above mentioned, it is still accompanied by problems since it is practiced under elevated temperature conditions. Other problems are presented in the process, including the apparatus to be used.
Regarding the former problems, because the plasma CVD method is practiced while maintaining a substrate at elevated temperature, the kind of the substrate to be used is limited to those which do not contain a material such as heavy metal, which can migrate and sometimes cause changes in the characteristics of the deposited film to be formed. Secondly, the substrate thickness is likely to be varied on standing in the plasma CVD method. Therefore, the resulting deposited film, lacking in uniformity of thickness and in homogeneity of composition, can exhibit changed characteristics.
Regarding the latter problems, the operation conditions to be employed under the plasma CVD method are much more complicated than the known CVD method, and it is extremely difficult to generalize them.
That is, there already exist a number of variations even in the correlated parameters concerning the temperature of a substrate, the amount and the flow rate of gases to be introduced, the degree of pressure and the high frequency power for forming a layer, the structure of an electrode, the structure of a reaction chamber, the rate of flow of exhaust gases, and the plasma generation system. Besides said parameters, there also exist other kinds of parameters. Under these circumstances, in order to obtain a desirable deposited film product, it is required to choose precise parameters from a great number of varied parameters. Sometimes, serious problems occur. Because of the precisely chosen parameters, a plasma is apt to be in an unstable state. This condition often invites problems in a deposited film to be formed.
And for the apparatus in which the process using the plasma CVD method is practiced, its structure will eventually become complicated since the parameters to be employed are precisely chosen as above stated. Whenever the scale or the kind of the apparatus to be used is modified or changed, the apparatus must be so structured as to cope with the precisely chosen parameters.
In this regard, even if a desirable deposited film should be fortuitously mass-produced, the film product becomes unavoidably costly because (1) a heavy initial investment is necessitated to set up a particularly appropriate apparatus therefor; (2) a number of process operation parameters even for such apparatus still exist and the relevant parameters must be precisely chosen from the existing various parameters for the mass-production of such film. In accordance with such precisely chosen parameters, the process must then be carefully practiced.
In order to prepare a desired functional A-Si film without the above problems, there has been proposed a method by way of the chemical reaction among gaseous raw material and active species or by way of the chemical reaction among active species using a fabrication apparatus as shown in FIG. 1 or another fabrication apparatus as shown in FIG. 2.
The fabrication apparatus of FIG. 1 comprises a film forming chamber 102, a film forming raw material gas transportation pipe 105, an active species (I) generation region 108 and a pipe portion 109 having a space B for mixing a film forming raw material gas and an active species (I) and transporting a gaseous mixture of them into the film forming chamber.
The film forming chamber 102 has a film forming space A in which a substrate holder 103 for substrate 104 having electric heater 110 being connected to a power source (not shown) by means of lead wires (not shown) is installed.
The film forming chamber 102 is provided with an exhaust pipe 101' connected through a main valve (not shown) to an exhaust pump 101, and the exhaust pipe is provided with a subsidiary valve (not shown) serving to break the vacuum in the film forming chamber 102. Numeral 111 stands for a vacuum gauge to monitor the inner pressure of the film forming space A.
The active species (I) generation region 108 comprises pipe portion 106 having an active species (I) generation space C with which a microwave energy applying applicator 107 is provided, and the microwave introducing applicator is connected to a microwave power source. To one end of the pipe portion 106, an active species (I) raw material gas (H.sub.2) feed pipe extended from a reservoir for said gas is connected. The other end of the pipe portion 106 is joined with the film forming raw material gas transportation pipe 105 at the upstream region of the pipe portion 106.
The pipe portion 106 is open at its downstream end in the film forming space A. To the end portion of the film forming raw material gas transportation pipe 105, a film forming raw material gas (SiF.sub.4) feed pipe extended from a reservoir for said gas is connected.
The fabrication apparatus of FIG. 2 is a partial modification of the fabrication apparatus of FIG. 1, and the modified part is that the film forming raw material gas transportation pipe 105 in FIG. 1 is replaced by an active species (II) generation region 201. In the fabrication apparatus of FIG. 2, the active species (II) generation region 201 comprises pipe portion 105 having an active species (II) generation space D with which a microwave energy applying applicator 202 is provided, and the microwave introducing applicator 202 is connected to a microwave power source (not shown).
The film forming process for preparing a A-Si thin film using the fabrication apparatus of FIG. 1 is carried out, for example, in the following way.
That is, the air in the film forming chamber, the film forming raw material gas transportation pipe 105 and the precursor generation space C is evacuated by opening the main valve of the exhaust pipe 101' to bring the chamber and other spaces to a desired vacuum. Then the heater 110 is activated to uniformly heat the substrate 104 to a desired temperature, and it is kept at this temperature. At the same time, SiF.sub.4 gas is fed at a desired flow rate into the transportation pipe 105 and then into the film forming space A through the space B. Concurrently, H.sub.2 gas is fed at a desired flow rate into the active species (I) generation space C and then into the film forming space A through the space B. After the flow rates of the two gases became stable, the vacuum of the film forming space A is brought to and kept at a desired value by regulating the main valve of the exhaust pipe 101'.
After the vacuum of the film forming space A becomes stable, the microwave power source is switched on to apply a discharge energy of a desired power into the active species (I) generation space C through the microwave energy applying applicator 107.
In this event, H.sub.2 gas is activated with the discharge energy to generate active species (I), which are successively flowed into the space B, then mixed with SiF.sub.4 gas flowed from the transportation pipe 105 therein, and transported into the film forming space A while being chemically reacted. The gaseous reaction mixture thus introduced is flowed in the space surrounding the surface of the substrate 104 being maintained at a desired temperature in the film forming space A and decomposed to thereby cause the formation of a A-Si:H:F thin film on the substrate.
The chemical reactions to cause the formation of said A-Si:H:F thin film in the case of the above process using the fabrication apparatus of FIG. 1 is considered to be progressed in the following ways.
(a) Reaction in the active species (I) generation space C:
H.sub.2 .fwdarw.2.multidot.H(.multidot.H means hydrogen radical) PA1 (i) .multidot.H+SiF.sub.4 .fwdarw.reaction product (1) PA1 (ii) .multidot.H+the reaction product I.fwdarw.reaction product (2) PA1 H.sub.2 .fwdarw.2 .multidot.H (.multidot.H means hydrogen radical) PA1 SiF.sub.4 .fwdarw.decomposed product such as SiF.sub.2.sup.*, SiF.sub.3.sup.*, Si.sup.*, SiF.sub.2, etc. (c) Reaction in the space B: PA1 .multidot.H+decomposed product.fwdarw.reaction product (3)
(b) Reaction in the space B:
The above-mentioned reaction product (1) is a molecule or a radical which are not sufficiently reduced and which are highly volatile, and it does not directly contribute to the formation of said A-Si:H:F thin film as it is. The above-mentioned reaction product (2) is a product resulting from the reaction product (1) being further reduced with the hydrogen radical (.H), and it contributes to the formation of said A-Si:H:F thin film on the substrate 104 in the film forming space A.
The exact mechanism of how the formation of said A--SiH:F thin film is caused onto the surface of the substrate is not clarified yet. However it is thought that the reaction product (2) as it flows into the film forming space A will be decomposed with the action of a thermal energy in the space surrounding the surface of the substrate being maintained with an elevated temperature and some of the reaction product (2) will collide against said surface of the substrate to thereby decompose into neutral radical particles, ion particles, electrons, etc. The chemical reactions among them will result in formation of said A-Si:H:F thin film on the surface of the substrate.
However, there still remain unsolved problems in the case of the above process using the fabrication apparatus of FIG. 1 That is, as mentioned in the above reactions (b), hydrogen radical (.multidot.H) as generated in the active species (I) generation space C will be consumed twice for the reaction (i) and for the reaction (ii) and because of this, the amount of the reaction product (2) to be produced in the space B depends upon the amount of the remaining hydrogen radical. However, in the case of the known process using the fabrication apparatus of FIG. 1, the amount of such hydrogen radical to be directed to forming the reaction product (2) is not sufficient so that the reaction product (2) cannot be sufficiently produced and because of this, it is almost impossible to stably and efficiently form a desired deposited film at a high deposition rate and with a high utilization efficiency of raw material gas.
In addition to this problem, there are also other problems for the process using the fabrication apparatus of FIG. 1. That is, H.sub.2 gas is not efficiently consumed to generate hydrogen radical in the active species (I) generation space C and as a result, the amount of the hydrogen radical to be generated therein eventually becomes insufficient. And the hydrogen radical as generated will often collide against the inner wall face having roughness and occasionally having foreign deposits thereon, and if such situation happens, the radical becomes deactivated to be neutral as it is transported to space B. Therefore, there often occurs changes in the amount of the hydrogen radical to arrive in the space B, which result in bringing about changes in the amonunts of the reaction product (1) and because of this, the deposition rate of a deposited film to be formed will be changed and it will become difficult to stably obtain a desired deposited film of an uniform film quality.
As for the film forming process for forming a A-Si:H:F thin film using the fabrication apparatus of FIG. 2, it is carried out in the same way as in the case of the above process using the fabrication apparatus except that a microwave discharge energy of a desired power is applied into the active species (II) generation space D to thereby generate active species (II) from SiF.sub.4.
The chemical reactions to cause the formation of said A-Si:H:F film in the case of the process using the fabrication apparatus of FIG. 2 is considered to be progressed in the following ways.
(a) Reaction in the active species (I) generation space C:
(b) Reaction in the active species (II) generation space D:
And the exact mechanism of how the formation of said A--Si:H:F thin film is caused onto the surface of the substrate is not clarified yet.
However, it is thought that the reaction product (3) as it flows into the film forming space A will be decomposed by the action of a heat energy in the space surrounding the surface of the substrate being maintained with an elevated temperature and some of the reaction product (3) will collide against said surface of the substrate to thereby decompose into neutral radical particles, ion particles, electrons, etc. The chemical reactions among them will result in the formation of said A-Si:H:F thin film on the surface of the substrate.
However, even for this process, there still remains unsolved problems. That is, undesired changes often occur on the deposition rate and because of this, it is difficult to stably and efficiently obtain a desired A-Si:H:F thin film of an uniform film quality with a high deposition rate and with a high raw material gas utilization efficiency. In addition to this problem, there are also other problems. That is, as well as in the case of process using the fabrication apparatus of FIG. 1, H.sub.2 gas is not efficiently consumed to generate hydrogen radical (.multidot.H) in the active species (I) generation space C and the amount of the hydrogen radical to be generated therein eventually becomes insufficient. And the hydrogen radical as generated will often collide against the inner wall face having roughness and occasionally having foreign deposits thereon and hence, while the radical is being transported to the space B' it becomes deactivated to be neutral. And there often occurs changes in the amount of the hydrogen radical to arrive in the space B, which results in bringing about changes in the amounts of the reaction products (3). Further, certain amount of the decomposed product will be left without being consumed in the above-mentioned reaction (c) and it is flowed into the film forming space A. Because of this, the decomposed product that flowed into the film forming space A will have an undesired influence on the resulting deposited film.
In view of the above, there is now an increased demand for providing an imposed process that makes it possible to stably and efficiently mass-produce a desirable A-Si thin film of good film quality which has a wealth of practically applicable characteristics with a high deposition rate and with a high raw material gas utilization efficiency. Besides silicon, there is a similar situation for other kinds of non-single crystal functional materials such as polycrystal silicon, silicon nitride, silicon-germanium, silicon carbide, and silicon oxide films.